RFR (S) 8245465: Shenandoah: test_in_cset can use more efficient encoding

[Roman Kennke]

Maybe this warrants a note why you cast to X and back, instead of using
the seemingly more obvious normal AddP addresing? Otherwise we may
scratch our heads in a year or so why we did this?

Roman

> RFE:
>   https://cr.openjdk.java.net/~shade/8245465/webrev.02/
> 
> See the rationale in the bug itself.
> 
> Webrev:
>   https://bugs.openjdk.java.net/browse/JDK-8245465
> 
> Testing: hotspot_gc_shenandoah, tier{1,2} with Shenandoah, eyeballing
> asm, benchmarks
>